1. Field of the Invention
The present invention relates to a metal oxide sintered structure used as a catalyst in gas modification, gas synthesis, desulfurization, combustion, or the like. In particular, it relates to a metal oxide sintered structure with a metal catalyst precipitated on the honeycomb supporter surface or the foam-like supporter surface formed by sintering a metal oxide-, and a production method therefore.
2. Description of the Related Art
Recently, fuel cells attract attention dramatically as a power generation technique with little carbon dioxide emission and environmental load. Conventionally, in the fuel cells, an enriched hydrogen gas obtained by modifying a material fuel such as a hydrocarbon and a methanol has been used.
As a method for producing the hydrogen as the fuel for the fuel cells, a carbon dioxide modifying method or a water vapor modifying method, of reacting a hydrocarbon gas and a carbon dioxide or a water vapor on a catalyst, is known. According to the method, a modified catalyst structure with a catalyst containing fine particles of Ni, Co, or the like as the main active substance supported on the surface of a sintered compact made of an oxide ceramic comprising an alumina, a magnesia, a silica, or the like is used. To the catalyst system (catalyst and supporter), various characteristics such as a high activity at a low temperature, a long life and a high heat conductivity are required.
Moreover, a sulfur component is included in a city gas, or the like as the material gas for producing the hydrogen fuel for the fuel cell, and thus a desulfurization process for eliminating the same is executed.
In the desulfurization process, a composite catalyst of Mo, W, Nb, or the like and Ni, Co, or the like is used in the same form as the above-mentioned modified catalyst. Also in the desulfurization catalyst, various characteristics are required in terms of the activity of the catalyst, the life, or the like.
Accordingly, in the catalyst used in the fuel modifying process of the fuel cell, characteristics such as homogeneous dispersion of the metal particles as the catalyst on the supporter for supporting the catalyst and adhesion of the supporter and the metal particles without the risk of desorption of the catalyst particles during the catalyst operation, are indispensable so that the dispersion and the adhesion state of the catalyst supporter and the catalyst particles are important elements for controlling the above-mentioned catalyst functions.
The conventional fuel modifying catalyst has been produced by forming catalyst metal particles on the surface of a ceramic sintered compact by the co-precipitation method, or the like. The co-precipitation method is for obtaining a catalyst system with the catalyst particles comprising the metal fine particles dispersed on the catalyst supporter by reducing the fine particles after precipitating fine particles containing the catalyst element on the catalyst supporter.
According to this method, although the particle of the catalyst metal can be made smaller, it is difficult to control the dispersion state. Furthermore, a problem is involved in that the interface strength between the precipitated fine particles and the supporter is weak and the adhesion property is poor so that the particles have the aggregation growth during the drive under a heating environment so as to deteriorate the catalyst efficiency.
Moreover, particularly in the case of using a structure having non-round channels (fluid communicating holes) such as a honeycomb, since the corner parts of the channels serve as the prioritized precipitation sites of the catalyst metal particles, the catalyst is produced unevenly on the channel wall surface so that a problem is involved in that the numerical density of the catalyst particles is low in the parts other than the corner parts.
According to the conventional method, due to the above-mentioned problems, one having a sufficiently satisfactory performance has not been developed in terms of the life, the activity, or the like.
Accordingly, since the above-mentioned problems in the conventional catalyst supporting structure production have not been solved, for example, a carbon dioxide modifying catalyst for the hydrogen as the fuel gas for a fuel cell or a water vapor modifying catalyst sufficiently satisfactory has not been obtained in terms of the catalyst performance. Furthermore, in the case of using as the desulfurization catalyst, in addition to the above-mentioned problems, a problem is involved in that the sulfur can hardly be eliminated.